Packet processing by all-optical signal is expected as a packet routing technique for a future photonic network. However, because of various problems, there is no prospect of practical use of the packet routing technique. For example, from the viewpoint of an optical amplifying relay device, there is the following technical problem.
In an optical burst communication or an optical packet communication, optical signals are transmitted in a state where the optical signals are not scrambled statistically, that is, in a burst state, resulting in a period with no optical signal. If the intermittent optical signals are input into the optical amplifier used in an optical relay device, a temporal change (an optical surge) may be generated in a rising part of an output light as illustrated in FIG. 1 due to a transient response of a gain in the optical amplifier. As a result, after being amplified, the optical signals may have wavelength deterioration. The problem is addressed by realizing an optical amplifier that amplifies the intermittent optical signals transmitted in the burst state while reducing the wavelength deterioration.
An optical method of using an optical loop circuit or the like (for example, see Non-Patent Document 1) and an electrical method of using an Automatic Gain Control (AGC) circuit or the like are reported as a conventional technique that addresses the above-described problem. However, there is a problem that the configuration of the optical method is complex and has low controllability. On the other hand, the electrical method is effective when the period with the optical signal and the period without the optical signal are switched with respect to each other at a relatively slow speed. However, if data such as an optical packet of which granularity is small is handled, the control speed of the electrical method is insufficient, and no response is made.
Apart from the above-described optical method and electrical method, there is a technique for controlling the transient response by using a rare earth-doped fiber of which an active region is expanded (for example, see Patent Document 1). Specifically, as illustrated in FIG. 2, by using the rare earth-doped fiber of which the active region is expanded, the non-saturation region according to the amplification property, which indicates a relation between an input power and the optical gain, is expanded. Due to this, the transient response becomes controllable because the optical gain is hardly changed even if the input power suddenly changes in a rising edge of the optical signal that is intermittently input.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2008-300818.
[Non-Patent Document 1] Chun-Liu Zhao, Hwa-Yaw Tam, Bai-Ou Guan, Xinyong Dong, P. K. A. Wai, Xiaoyi Dong, “Optical automatic gain control of EDFA using two oscillating lasers in a single feedback loop”, Optics Communications, Volume 225, Issues 1-3, 15 Sep. 2003, pp. 157-162.
[Non-Patent Document 2] Cechan Tian, Susumu Kinoshita, “Analysis and Control of Transient Dynamics of EDFA Pumped by 1480- and 980-nm Lasers”, Journal of Lightwave Technology, Vol. 21, No. 8, August 2003, pp. 1728-1734.
[Non-Patent Document 3] Haruo Nakaji, Yoshiharu Nakai, Masayuki Shigematsu, Masayuki Nishimura, “Superior high-speed automatic gain controlled erbium-doped fiber amplifiers”, Optical Fiber Technology, Volume 9, Issue 1, January 2003, pp. 25-35.
However, regarding the optical amplifier that suppresses the transient response by applying the above-described conventional technique, there is a problem that a deflection between wavelengths is generated in the output light power if a Wavelength Division Multiplexing (WDM) light is handled. That is, when a plurality of optical signals with various wavelengths is collectively amplified by the optical amplifier, for example, the gain on the shorter wavelength side illustrated in FIG. 3A relatively decreases, and a wavelength tilt is generated in the optical signal power of each wavelength output from the optical amplifier if the total power of the optical signal of each wavelength becomes large and optical amplification operation is performed across the saturation region.
In the above-described Patent Document 1, the optical signal power of each wavelength after being amplified is equalized by using a gain equalizing filter. However, if setting of a desired gain in the optical amplifier is changed due to change of an operation state of the WDM light, a wavelength property of the gain changes. Therefore, the wavelength tilt is unlikely to be controlled sufficiently by the gain equalization filter of which a transparent property is fixed. If the transparent property of the gain equalization filter is actively controlled, the output optical power may be equalized. However, the gain equalization filter of which the transparent property is variable is expensive, and the control of the transparent property is complex. This may disturb the practical use of the gain equalization filter.
For the control of the optical amplifier corresponding to the related WDM light, as illustrated in FIG. 3B, there is a known technique for controlling generation of the wavelength tilt by applying the AGC to equalize the gain regardless of the change of the input power. As illustrated in FIG. 4, in the optical amplifier that applies the AGC, the optical amplification operation is stopped when a monitor value of the total power of the input light is equal to or smaller than an input break detection threshold value that is set in advance. This is because the accuracy of a monitor decreases because noise generated in a circuit part monitoring the input light is dominant, and a control error of the AGC is generated, and stability to temperature variation becomes inefficient. If the AGC used in this way is applied to the amplification of optical signals with various wavelengths that are intermittently input, the signal densities of the input lights are temporally different from each other. Thus, in a period in which the signal density is low, the value obtained by monitoring the total power of the input light in a prescribed period decreases even though the signal level is normal. Accordingly, if the monitor value is equal to or smaller than the input break detection threshold value, the optical amplification operation is stopped even though the intermittent optical signals are normally input.